Whether harvesting crops on Farmville or building a dream metropolis on CityVille, chances are you use Facebook Credits. Facebook is expanding its virtual currency, making the use of Facebook Credits mandatory for developers starting July 1.

Facebook credits are the virtual currency used to purchase premium goods on the social-networking site.

"With Facebook Credits, people enter their payment information once and can buy, earn, and spend safely across lots of different games," Facebook's Deborah Liu wrote in a blog post.

Facebook has been ramping up the credits program for quite some time, and added additional payment options via monetization service PlaySpan in October.

Through credits, Facebook takes 30 percent of all transactions and awards 70 percent to developers. According to the Wall Street Journal, this is a pretty big cut for the social-networking site to take. By contrast, PayPal only charges three percent on transactions.

That said, Liu noted that Facebook Credits already account for 70 percent of transactions on the site and are used in more than 350 apps from 150 different developers. The service is used by game developers like PlayFish, CrowdStar, PopCap, and Digital Chocolate. Both Electronic Arts and Zynga have signed five-year agreements to make credits the exclusive payment option in their games on the site.

"Over the next five months, we will work closely with developers to onboard those who are not yet using Facebook Credits, collect feedback to improve the product, continue to innovate on the user experience, and help developers grow their revenue on Facebook," Liu said.